Process monitoring of deep structures with X-ray scatterometry

ABSTRACT

Methods and systems for estimating values of process parameters, structural parameters, or both, based on x-ray scatterometry measurements of high aspect ratio semiconductor structures are presented herein. X-ray scatterometry measurements are performed at one or more steps of a fabrication process flow. The measurements are performed quickly and with sufficient accuracy to enable yield improvement of an on-going semiconductor fabrication process flow. Process corrections are determined based on the measured values of parameters of interest and the corrections are communicated to the process tool to change one or more process control parameters of the process tool. In some examples, measurements are performed while the wafer is being processed to control the on-going fabrication process step. In some examples, X-ray scatterometry measurements are performed after a particular process step and process control parameters are updated for processing of future devices.

CROSS REFERENCE TO RELATED APPLICATION

The present application for patent claims priority under 35 U.S.C. § 119from U.S. provisional patent application Ser. No. 62/512,297, entitled“Process Monitoring for Deep Structures Using X-Ray Scatterometry,”filed May 30, 2017 and from U.S. provisional patent application Ser. No.62/572,566, entitled “Process Monitoring for Deep Structures Using X-RayScatterometry,” filed Oct. 16, 2017, the subject matter of each isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The described embodiments relate to metrology systems and methods, andmore particularly to methods and systems for improved measurement ofsemiconductor structures undergoing a fabrication process step.

BACKGROUND INFORMATION

Semiconductor devices such as logic and memory devices are typicallyfabricated by a sequence of processing steps applied to a specimen. Thevarious features and multiple structural levels of the semiconductordevices are formed by these processing steps. For example, lithographyamong others is one semiconductor fabrication process that involvesgenerating a pattern on a semiconductor wafer. Additional examples ofsemiconductor fabrication processes include, but are not limited to,chemical-mechanical polishing, etch, deposition, and ion implantation.Multiple semiconductor devices may be fabricated on a singlesemiconductor wafer and then separated into individual semiconductordevices.

Metrology processes are used at various steps during a semiconductormanufacturing process to detect defects on wafers to promote higheryield. A number of metrology based techniques including scatterometryand reflectometry implementations and associated analysis algorithms arecommonly used to characterize critical dimensions, film thicknesses,composition and other parameters of nanoscale structures. X-rayscatterometry techniques offer the potential for high throughput withoutthe risk of sample destruction.

Traditionally, optical scatterometry critical dimension (SCR)measurements are performed on targets consisting of thin films and/orrepeated periodic structures. During device fabrication, these films andperiodic structures typically represent the actual device geometry andmaterial structure or an intermediate design. As devices (e.g., logicand memory devices) move toward smaller nanometer-scale dimensions,characterization becomes more difficult. Devices incorporating complexthree-dimensional geometry and materials with diverse physicalproperties contribute to characterization difficulty. For example,modern memory structures are often high-aspect ratio, three-dimensionalstructures that make it difficult for optical radiation to penetrate tothe bottom layers. Optical metrology tools utilizing infrared to visiblelight can penetrate many layers of translucent materials, but longerwavelengths that provide good depth of penetration do not providesufficient sensitivity to small anomalies. In addition, the increasingnumber of parameters required to characterize complex structures (e.g.,FinFETs), leads to increasing parameter correlation. As a result, theparameters characterizing the target often cannot be reliably decoupledwith available measurements.

In one example, longer wavelengths (e.g. near infrared) have beenemployed in an attempt to overcome penetration issues for 3D FLASHdevices that utilize polysilicon as one of the alternating materials inthe stack. However, the mirror like structure of 3D FLASH intrinsicallycauses decreasing light intensity as the illumination propagates deeperinto the film stack. This causes sensitivity loss and correlation issuesat depth. In this scenario, optical SCD is only able to successfullyextract a reduced set of metrology dimensions with high sensitivity andlow correlation.

In another example, opaque, high-k materials are increasingly employedin modern semiconductor structures. Optical radiation is often unable topenetrate layers constructed of these materials. As a result,measurements with thin-film scatterometry tools such as ellipsometers orreflectometers are becoming increasingly challenging.

In response to these challenges, more complex optical metrology toolshave been developed. For example, tools with multiple angles ofillumination, shorter illumination wavelengths, broader ranges ofillumination wavelengths, and more complete information acquisition fromreflected signals (e.g., measuring multiple Mueller matrix elements inaddition to the more conventional reflectivity or ellipsometric signals)have been developed. However, these approaches have not reliablyovercome fundamental challenges associated with measurement of manyadvanced targets (e.g., complex 3D structures, structures smaller than10 nm, structures employing opaque materials) and measurementapplications (e.g., line edge roughness and line width roughnessmeasurements).

Optical methods may provide non-destructive tracking of process variablebetween process steps, but regular calibration by destructive methods isrequired to maintain accuracy in the face of process drift, whichoptical methods cannot independently distinguish.

Atomic force microscopes (AFM) and scanning-tunneling microscopes (STM)are able to achieve atomic resolution, but they can only probe thesurface of the specimen. In addition, AFM and STM microscopes requirelong scanning times. Scanning electron microscopes (SEM) achieveintermediate resolution levels, but are unable to penetrate structuresto sufficient depth. Thus, high-aspect ratio holes are not characterizedwell. In addition, the required charging of the specimen has an adverseeffect on imaging performance. X-ray reflectometers also suffer frompenetration issues that limit their effectiveness when measuring highaspect ratio structures.

To overcome penetration depth issues, traditional imaging techniquessuch as TEM, SEM etc., are employed with destructive sample preparationtechniques such as focused ion beam (FIB) machining, ion milling,blanket or selective etching, etc. For example, transmission electronmicroscopes (TEM) achieve high resolution levels and are able to probearbitrary depths, but TEM requires destructive sectioning of thespecimen. Several iterations of material removal and measurementgenerally provide the information required to measure the criticalmetrology parameters throughout a three dimensional structure. But,these techniques require sample destruction and lengthy process times.The complexity and time to complete these types of measurementsintroduces large inaccuracies due to drift of etching and metrologysteps because the measurement results become available long after theprocess has been completed on the wafer under measurement. Thus, themeasurement results are subject to biases from further processing anddelayed feedback. In addition, these techniques require numerousiterations which introduce registration errors. In summary, device yieldis negatively impacted by long and destructive sample preparationrequired for SEM and TEM techniques.

In semiconductor device manufacturing, etch processes and depositionprocesses are critical steps to define a device pattern profile andlayout on a semiconductor wafer. Thus, it is important to measure filmsand patterned structures to ensure the fidelity of the measuredstructures and their uniformity across the wafer. Furthermore, it isimportant to provide measurement results quickly to control the on-goingprocess and to adjust settings to maintain required pattern or filmuniformity across the wafer.

In most examples, precise monitoring of a semiconductor manufacturingprocess is performed by one or more stand-alone (SA) metrology systems.SA metrology systems usually provide the highest measurementperformance. However, the wafer must be removed from the process toolfor measurement. For processes undertaken in vacuum, this causessignificant delay. As a result, SA metrology systems cannot provide fastmeasurement feedback to process tools, particularly process toolsinvolving vacuum. In other examples, integrated metrology systems orsensors are often attached to process equipment to measure wafers aftera process step is completed, but without removing the wafer from theprocess tool. In other examples, in-situ (IS) metrology systems orsensors are employed inside a processing chamber of a process tool.Furthermore, an IS metrology system monitors the wafer during theprocess (e.g., etch process, deposition process, etc.) and providesfeedback to the process tool performing the fabrication step undermeasurement.

In one example, structures subject to a reactive ion etch process aremonitored in-situ. In some fabrication steps, the etch process isrequired to etch completely through an exposed layer and then terminatebefore substantial etching of a lower layer occurs. Typically, theseprocess steps are controlled by monitoring the spectral signature of theplasma present in the chamber using an emission spectroscopy technique.When the exposed layer is etched through and the etch process begins toreact with a lower layer, a distinct change in the spectral signature ofthe plasma occurs. The change in spectral signature is measured by theemission spectroscopy technique, and the etch process is halted based onthe measured change is spectral signature.

In other fabrication steps, the etch process is required to etchpartially through an exposed layer to a specified etch depth, andterminate before etching completely through the exposed layer. This typeof etch process is commonly referred to as a “blind etch”. Currently,the measurement of etch depth through partially etched layers is basedon near-normal incidence spectral reflectometry.

Current in-situ sensors are only capable of monitoring bulk changes tofilm thicknesses and do not correlate well to the complex profiles thatresult from the processing of deep 3-D structures.

In general, there are many methods of process monitoring usingcombinations of optical, acoustic and electron beam tools. Thesetechniques measure the device directly, specially designed targets, orspecific monitor wafers. However, the inability to measured parametersof interest of high aspect ratio structures in a cost effective andtimely manner results in low yield, particularly in the memory sector ofa wafer.

In summary, ongoing reductions in feature size and increased depth ofmany semiconductor structures imposes difficult requirements onmetrology systems, including stand-alone systems and those integratedwith process tools, such as ion implant and etch tools. Thus, improvedmetrology systems and methods are desired to measure high aspect ratiostructures to maintain high device yield.

SUMMARY

Methods and systems for estimating values of process parameters,structural parameters, or both, based on x-ray scatterometrymeasurements of high aspect ratio semiconductor structures are presentedherein. X-ray scatterometry measurements are performed at one or moresteps of a fabrication process flow. Exemplary process steps includeetch, deposition, and lithography processes. The measurements areperformed quickly and with sufficient accuracy to enable yieldimprovement of an on-going semiconductor fabrication process flow. Insome examples, measurements are performed while the wafer is beingprocessed to control the on-going fabrication process.

In one aspect, a small-angle scatterometry (SAXS) metrology system isintegrated with a wafer processing tool, and measured values ofparameters of interest are provided as feedback to control the waferprocessing tool. In some embodiments, a transmission SAXS measurementsystem is employed. In other embodiments, a reflective SAXS measurementsystem is employed.

In some embodiments, a SAXS metrology system is integrated with a waferprocessing tool such as an etch, deposition, or lithography tool. Inother embodiments, the SAXS metrology system is constructed as astand-alone tool.

In a further aspect, a SAXS system estimates values of one or moreparameters of interest (e.g., process parameter values, structuralparameter values, or both) based on scatterometry measurements of wafersunder process using a measurement model.

In another further aspect, process corrections are determined based onthe measured values of the parameters of interest and the correctionsare communicated to the process tool to change one or more processcontrol parameters of the process tool. In some embodiments, SAXSmeasurements are performed and process control parameters are updatedwhile the process is being executed on the measured structure. In someembodiments, SAXS measurements are performed after a particular processstep and process control parameters associated with that process stepare updated for processing of future devices by that process step. Insome embodiments, SAXS measurements are performed after a particularprocess step and process control parameters associated with a subsequentprocess step are updated for processing of the measured device or otherdevices by the subsequent process step.

The measurement frequency of a particular high aspect ratio structureunder fabrication depends on the stability of the monitored processparameter. Moreover, the length of measurement time required depends onthe scattering sensitivity to changes in the monitored process variable.Measuring a scattering signal in-situ provides the fastest measurementof process conditions but with the highest degree of uncertainty.Whereas, longer measurement times provide greater precision andcertainty of the measured parameters. In general, process parametersthat affect the entire scattering volume (e.g., etch time) can bemonitored the fastest, while other parameters (e.g., minor deviations ofthe etch profile) require either a moving average or longer measurementtime to achieve meaningful results. Thus, these parameters can only becontrolled on a slower basis.

The foregoing is a summary and thus contains, by necessity,simplifications, generalizations and omissions of detail; consequently,those skilled in the art will appreciate that the summary isillustrative only and is not limiting in any way. Other aspects,inventive features, and advantages of the devices and/or processesdescribed herein will become apparent in the non-limiting detaileddescription set forth herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an exemplary wafer processing system 100 for monitoringof an etch process based on x-ray scatterometry measurements ofsemiconductor structures disposed on a wafer under process.

FIG. 2 is a diagram illustrative of an x-ray illumination beam incidenton a wafer at a particular orientation described by an angle ofincidence, θ, and an azimuth angle, ϕ.

FIG. 3 is a diagram illustrative of a semiconductor structure includingtwo hole features undergoing an etch process in one embodiment.

FIG. 4 is a diagram illustrative of a semiconductor structure includingtwo hole features undergoing a deposition process in another embodiment.

FIG. 5 is a diagram illustrative of a semiconductor structure includingtwo layers, each including two hole features.

FIG. 6 depicts scattering images illustrative of changes of size of anetched hole.

FIG. 7 depicts scattering images illustrative of changes of the depth ofan etched hole.

FIG. 8 depicts a plot of the scattering efficiency of the zero orderbeam, S₀₀, as a function of angle of incidence, θ.

FIG. 9 depicts the scattering efficiency of several higher orders as afunction of angle of incidence, θ.

FIG. 10 depicts a top view of an array of high aspect ratio holestructures.

FIG. 11A is a diagram illustrative of a side view of an ideal highaspect ratio hole structure.

FIG. 11B is a diagram illustrative of a side view of a tilted holestructure.

FIG. 11C is a diagram illustrative of a side view of a progressivelytilted hole structure.

FIGS. 12A-12C depict an isometric view, a top view, and across-sectional view, respectively, of a typical 3D FLASH memory devicesubject to measurement as described herein.

FIG. 13 depicts an exemplary wafer processing system 200 for monitoringof an etch process based on reflective x-ray scatterometry measurementsof semiconductor structures disposed on a wafer under process.

FIG. 14 is a diagram illustrative of an exemplary model building andanalysis engine 180.

FIG. 15 illustrates a flowchart of a method 300 for controlling asemiconductor fabrication process for high aspect ratio structures basedon small angle X-Ray scatterometry measurements.

DETAILED DESCRIPTION

Reference will now be made in detail to background examples and someembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

Methods and systems for estimating values of process parameters,structural parameters, or both, based on x-ray scatterometrymeasurements of partially fabricated, high aspect ratio semiconductorstructures are presented herein. X-ray scatterometry measurements ofhigh aspect ratio structures are performed at one or more steps of afabrication process flow. Exemplary process steps include etch,deposition, and lithography processes. The measurements are performedquickly and with sufficient accuracy to enable yield improvement of anon-going semiconductor fabrication process flow. In some examples,measurements performed while the wafer is being processed are used tocontrol the on-going process. High aspect ratio structures includesufficient overall scattering volume and material contrast toefficiently scatter incident x-rays. The collected, scattered x-raysenable accurate estimation of structural parameters of interest ofmeasured devices. The x-ray energy is high enough to penetrate thesilicon wafer and process gases in the optical path with minimal signalcontamination.

Device yield of advanced semiconductor manufacturing nodes continues tosuffer, particularly the device yield of complex, high aspect ratio(deep, three-dimensional) structures. Real-time monitoring and processcontrol based on x-ray scatterometry enables process control for thefabrication of high aspect ratio structures in a cost effective mannercompared to traditional destructive techniques such as SEM, TEM, etc.

X-ray scatterometry measurements provide accurate estimates ofstructural parameters of interest of high aspect ratio structures athigh throughput without destroying the sample under measurement.Measurement sensitivity is not significantly affected by penetrationdepth, enabling accurate measurement of structures located deep withinthe vertical stack of the measured semiconductor structure. Moreover,x-ray radiation propagating through a plasma processing environment isrelatively insensitive to signal contamination from electromagneticfields generated by the plasma process compared to optical radiation.

In one aspect, a small-angle scatterometry (SAXS) metrology system isintegrated with a wafer processing tool, and measured values ofparameters of interest are provided as feedback to control the waferprocessing tool.

FIG. 1 depicts an exemplary wafer processing system 100 for monitoringof an etch process based on x-ray scatterometry measurements ofsemiconductor structures disposed on a wafer under process. In thedepicted embodiment, a transmission, small-angle scatterometry (T-SAXS)metrology system is integrated with an etch process tool. The measuredvalues of the parameters of interest are provided as feedback to controlthe etch process tool.

Wafer processing system 100 includes a process chamber 104 containing aprocess environment 103 and an x-ray scatterometer. Semiconductor wafer101 is located within process chamber 104. Wafer 101 is attached towafer chuck 105 and is positioned with respect to process chamber 104and the x-ray scatterometer by wafer stage 140.

In some embodiments, wafer stage 140 moves wafer 101 in the XY plane bycombining a rotational movement with a translational movement (e.g., atranslational movement in the X direction and a rotational movementabout the Y-axis) to position wafer 101 with respect to the illuminationprovided by the x-ray scatterometer. In some other embodiments, waferstage 140 combines two orthogonal, translational movements (e.g.,movements in the X and Y directions) to position wafer 101 with respectto the illumination provided by the x-ray scatterometer. In someembodiments, wafer stage 140 is configured to control the position ofwafer 101 with respect to the illumination provided by the x-rayscatterometer in six degrees of freedom. In general, specimenpositioning system 140 may include any suitable combination ofmechanical elements to achieve the desired linear and angularpositioning performance, including, but not limited to goniometerstages, hexapod stages, angular stages, and linear stages.

In some embodiments, wafer processing system 100 does not include waferstage 140. In these embodiments, a wafer handling robot (not shown)locates wafer 101 on wafer chuck 105 inside process chamber 104. Wafer101 is transferred from the wafer handling robot onto an electrostaticwafer chuck 105 that is compatible with a vacuum process environment103. In these embodiments, the measurements performed by the x-rayscatterometer are limited to the portion of wafer 101 within the fieldof view of the x-ray scatterometer after clamping of wafer 101 ontowafer chuck 105. In this sense, wafer stage 140 is optional. To overcomethis limitation, wafer processing system 100 may include multiple x-rayscatterometer systems, each measuring a different area of wafer 101.

In one embodiment, process chamber 104 is an element of a reactive ionetch system. In this embodiment, process environment 103 includes aradio frequency induced plasma that etches away exposed material on thesurface of wafer 101.

As depicted in FIG. 1, the optical elements of the x-ray scatterometerare located outside of the process chamber 104. Ionized particles arepresent in the process chamber of both etch and deposition processes.Optical elements must be located sufficiently far away from the wafer toavoid disturbing the magnetic fields induced by the process. Inaddition, ionized particles may accumulate on optical elements locatedin the process chamber, and thus it is not practical to include theoptical elements in the process chamber.

In the depicted embodiment, the SAXS metrology system includes an x-rayillumination subsystem 125 including an x-ray illumination source 110,focusing optics 111, beam divergence control slit 112, intermediate slit113, and a beam shaping slit mechanism 120. The x-ray illuminationsource 110 is configured to generate x-ray radiation suitable for T-SAXSmeasurements. In some embodiments, the x-ray illumination source 110 isconfigured to generate wavelengths between 0.01 nanometers and 1nanometer. In general, any suitable high-brightness x-ray illuminationsource capable of generating high brightness x-rays at flux levelssufficient to enable high-throughput, inline metrology may becontemplated to supply x-ray illumination for T-SAXS measurements. Insome embodiments, an x-ray source includes a tunable monochromator thatenables the x-ray source to deliver x-ray radiation at different,selectable wavelengths.

In some embodiments, one or more x-ray sources emitting radiation withphoton energy greater than 15 keV, or greater than 17 keV, are employedto ensure that the x-ray source supplies light at wavelengths that allowsufficient transmission through the entire device as well as the wafersubstrate, and any intervening elements. Intervening elements mayinclude one or more windows (e.g., windows made from beryllium,sapphire, diamond, etc.). Intervening elements may also includestructures in the path of the scattered x-ray radiation between thewafer 101 and detector 119, such as wafer chuck 105, a load port, orelements of stage 140. Transmission through structural plastic materialsdoes not risk excessive contamination of the scattered signals.Apertures or windows through structural elements of wafer chuck 105,stage 140, or a load port may be employed to minimize signalcontamination. For example, the x-ray spot at the wafer may be as smallas 50-200 micrometers. For elements located close to the wafer, the sizeof the aperture required to minimize contamination of the scatteredorders is minimal. However, the required aperture size increases as thedistance from the wafer increases due to the finite scattering anglesassociated with the scattered orders of interest.

Exemplary x-ray sources include electron beam sources configured tobombard solid or liquid targets to stimulate x-ray radiation. Methodsand systems for generating high brightness, liquid metal x-rayillumination are described in U.S. Pat. No. 7,929,667, issued on Apr.19, 2011, to KLA-Tencor Corp., the entirety of which is incorporatedherein by reference.

By way of non-limiting example, x-ray illumination source 110 mayinclude any of a particle accelerator source, a liquid anode source, arotating anode source, a stationary, solid anode source, a microfocussource, a microfocus rotating anode source, a plasma based source, andan inverse Compton source. In one example, an inverse Compton sourceavailable from Lyncean Technologies, Inc., Palo Alto, Calif. (USA) maybe contemplated. Inverse Compton sources have an additional advantage ofbeing able to produce x-rays over a range of photon energies, therebyenabling the x-ray source to deliver x-ray radiation at different,selectable wavelengths.

In some examples, computing system 130 communicates command signals 137to x-ray illumination source 110 that cause x-ray illumination source110 to emit x-ray radiation at a desired energy level. The energy levelis changed to acquire measurement data with more information about thehigh aspect ratio structures under measurement.

X-ray illumination source 110 produces x-ray emission over a source areahaving finite lateral dimensions (i.e., non-zero dimensions orthogonalto the beam axis. Focusing optics 111 focuses source radiation onto ametrology target located on specimen 101. The finite lateral sourcedimension results in finite spot size 102 on the target defined by therays 117 coming from the edges of the source. In some embodiments,focusing optics 111 includes elliptically shaped focusing opticalelements.

A beam divergence control slit 112 is located in the beam path betweenfocusing optics 111 and beam shaping slit mechanism 120. Beam divergencecontrol slit 112 limits the divergence of the illumination provided tothe specimen under measurement. An additional intermediate slit 113 islocated in the beam path between beam divergence control slit 112 andbeam shaping slit mechanism 120. Intermediate slit 113 providesadditional beam shaping. In general, however, intermediate slit 113 isoptional.

Beam shaping slit mechanism 120 is located in the beam path beforespecimen 101. In some embodiments, beam shaping slit mechanism 120includes multiple, independently actuated beam shaping slits. In oneembodiment, beam shaping slit mechanism 120 includes four independentlyactuated beam shaping slits. These four beams shaping slits effectivelyblock a portion of incoming beam 115 and generate an illumination beam116 having a box shaped illumination cross-section.

In general, x-ray optics shape and direct x-ray radiation to specimen101. In some examples, the x-ray optics include an x-ray monochromatorto monochromatize the x-ray beam that is incident on the specimen 101.In some examples, the x-ray optics collimate or focus the x-ray beamonto measurement area 102 of specimen 101 to less than 1 milliradiandivergence using multilayer x-ray optics. In these examples, themultilayer x-ray optics function as a beam monochromator, also. In someembodiments, the x-ray optics include one or more x-ray collimatingmirrors, x-ray apertures, x-ray beam stops, refractive x-ray optics,diffractive optics such as zone plates, Montel optics, specular x-rayoptics such as grazing incidence ellipsoidal mirrors, polycapillaryoptics such as hollow capillary x-ray waveguides, multilayer optics orsystems, or any combination thereof. Further details are described inU.S. Patent Publication No. 2015/0110249, the content of which isincorporated herein by reference it its entirety.

In some embodiments, x-ray illumination source 110, focusing optics 111,slits 112 and 113, or any combination thereof, are maintained in acontrolled atmospheric environment (e.g., gas purge environment).However, in some embodiments, the optical path length between and withinany of these elements is long and x-ray scattering in air contributesnoise to the image on the detector. Hence in some embodiments, any ofx-ray illumination source 110, focusing optics 111, and slits 112 and113 are maintained in a localized, vacuum environment. In the embodimentdepicted in FIG. 1, focusing optics 111, slits 112 and 113, and beamshaping slit mechanism 120 are maintained in a controlled environment(e.g., vacuum) within an evacuated flight tube 118. The illuminationbeam 116 passes through window 122 at the end of flight tube 118 beforeincidence with window 106 of process chamber 104.

In some embodiments, flight tube 118 is integrated with process chamber104 with a window separating the process environment 103 from the vacuumenvironment maintained within flight tube 118.

After incidence with wafer 101, scattered x-ray radiation 114 exitsprocess chamber 104 through window 107. In some embodiments, the opticalpath length between process chamber 104 and detector 119 (i.e., thecollection beam path) is long and x-ray scattering in air contributesnoise to the image on the detector. Hence, in preferred embodiments, asignificant portion of the collection beam path length between processchamber 104 and detector 119 is maintained in a localized vacuumenvironment separated from the environment by a vacuum window (e.g.,vacuum window 124). In some embodiments, vacuum chamber 123 isintegrated with process chamber 104 with a window separating the processenvironment 103 from the vacuum environment maintained within vacuumchamber 123. In some embodiments, x-ray detector 119 is maintained inthe same localized vacuum environment as the beam path length betweenprocess chamber 104 and detector 119. For example, as depicted in FIG.1, vacuum chamber 123 maintains a localized vacuum environmentsurrounding detector 119 and a significant portion of the beam pathlength between process chamber 104 and detector 119.

In some other embodiments, x-ray detector 119 is maintained in acontrolled atmospheric environment (e.g., gas purge environment). Thismay be advantageous to remove heat from detector 119. However, in theseembodiments, it is preferable to maintain a significant portion of thebeam path length between process chamber 104 and detector 119 in alocalized vacuum environment within a vacuum chamber. In general, thevacuum windows may be constructed of any suitable material that issubstantially transparent to x-ray radiation (e.g., Kapton, Beryllium,etc.).

In the embodiment depicted in FIG. 1, illumination light passes throughone or more window elements 106 of gas injector system 108 of processchamber 104. Gas injector system 108 extends from window element 106into process chamber 104. In one embodiment, the distance from windowelement 106 and wafer 101 is approximately 300 millimeters and gasinjector system 108 extends approximately 150 millimeters from windowelements 106 toward wafer 101. Gas injector system 108 introduces a gasflow along the x-ray illumination path to prevent ionized gas particlesfrom impacting and contaminating window elements 106. Exemplary gasinjector systems are manufactured by LAM Research Corporation, Fremont,Calif. (USA).

X-ray detector 119 collects x-ray radiation 114 scattered from specimen101 and generates an output signals 135 indicative of properties ofspecimen 101 that are sensitive to the incident x-ray radiation inaccordance with a T-SAXS measurement modality. In some embodiments,scattered x-rays 114 are collected by x-ray detector 119 while specimenpositioning system 140 locates and orients specimen 101 to produceangularly resolved scattered x-rays.

In some embodiments, a T-SAXS system includes one or more photoncounting detectors with high dynamic range (e.g., greater than 10⁵). Insome embodiments, a single photon counting detector detects the positionand number of detected photons.

In some embodiments, the x-ray detector resolves one or more x-rayphoton energies and produces signals for each x-ray energy componentindicative of properties of the specimen. In some embodiments, the x-raydetector 119 includes any of a CCD array, a microchannel plate, aphotodiode array, a microstrip proportional counter, a gas filledproportional counter, a scintillator, or a fluorescent material.

In this manner the X-ray photon interactions within the detector arediscriminated by energy in addition to pixel location and number ofcounts. In some embodiments, the X-ray photon interactions arediscriminated by comparing the energy of the X-ray photon interactionwith a predetermined upper threshold value and a predetermined lowerthreshold value. In one embodiment, this information is communicated tocomputing system 130 via output signals 135 for further processing andstorage (e.g., in memory 190).

In a further aspect, a T-SAXS system is employed to determine propertiesof a specimen (e.g., structural parameter values) based on one or morediffraction orders of scattered light. As depicted in FIG. 1, system 100includes a computing system 130 employed to acquire signals 135generated by detector 119 and determine properties of the specimen basedat least in part on the acquired signals and store the determinedparameters of interest 122 in a memory (e.g., memory 190). In someembodiments, computing system 130 is configured as a process controlmetrology engine to directly estimate values of one or more parametersof interest based on scatterometry measurements of wafers under processusing a measurement model.

In another aspect, metrology based on T-SAXS involves determining thedimensions of the sample by the inverse solution of a pre-determinedmeasurement model with the measured data. The measurement model includesa few (on the order of ten) adjustable parameters and is representativeof the geometry and optical properties of the specimen and the opticalproperties of the measurement system. The method of inverse solveincludes, but is not limited to, model based regression, tomography,machine learning, or any combination thereof. In this manner, targetprofile parameters are estimated by solving for values of aparameterized measurement model that minimize errors between themeasured scattered x-ray intensities and modeled results.

In some embodiments, the measurement model is an electromagnetic model(e.g., a Born Wave Model) of the measurement that generates imagesrepresentative of the scattering from the target under measurement. Forexample, images 150-152 depicted in FIGS. 6 an 7 are imagesrepresentative of scattering from a target under measurement. Themodelled images may be parameterized by process control parameters(e.g., etch time, etch tilt, etch selectivity, deposition rate, etc.).The modelled images may also be parameterized by structural parametersof the measured high aspect ratio structure (e.g., height, diameter atdifferent heights, alignment of a hole with respect to other structures,the straightness of a hole feature, the concentricity of a hole feature,thickness of deposited layers as a function of depth, uniformity ofdeposited layers across a particular hole feature or between differenthole features, etc.).

The measured scattering images are employed to monitor a fabricationprocess by performing an inverse solve to estimate values of one or moreof the parameters of interest. In these examples, an inverse solve wouldsolve for values of process parameters, geometric parameters, or both,that generate modelled scattering images that most closely matchmeasured images. In some examples, the space of scattering images issearched using the measurement model using regression methods (e.g.,gradient descent, etc.). In some examples, a library of precomputedimages is generated and the library is searched to find values of one ormore of the parameters of interest that result in the best match betweenmodelled and measured images.

In some other examples, a measurement model is trained by a machinelearning algorithm to relate many samples of scattering images and knownprocess conditions, geometric parameter values, or both. In this manner,the trained measurement model maps measured scattering images toestimated values of process parameters, geometric parameters, or both.In some examples, the trained measurement model is a signal responsemetrology (SRM) model that defines a direct, functional relationshipbetween actual measurements and parameters of interest.

In general, any of the trained models described herein is implemented asa neural network model. In other examples, any of the trained models maybe implemented as a linear model, a non-linear model, a polynomialmodel, a response surface model, a support vector machines model, adecision tree model, a random forest model, a deep network model, aconvolutional network model, or other types of models.

In some examples, any of the trained models described herein may beimplemented as a combination of models. Additional description of modeltraining and the use of trained measurement models for semiconductormeasurements is provided in U.S. Patent Publication No. 2016/0109230 byPandev et al., the content of which is incorporated herein by referencein its entirety.

In some other examples, a free-form model that does not include apreconceived geometry and material distribution describes the geometryand material parameters of the structure under measurement. In someexamples, the model includes many small voxels (volumetric elements)that each have an independently adjustable material parameter value(e.g., electron density, absorptivity, or complex refractive index). Insome other embodiments, the material properties are piecewise constant.The properties associated with each different material are determined apriori. The boundaries between different materials are free-formsurfaces, and these surfaces can be determined by the level setalgorithm.

The measured scatterometry data is used to calculate an image of thesample. In some examples, the image is a two dimensional (2-D) map ofelectron density, absorptivity, complex index of refraction, or acombination of these material characteristics. In some examples, theimage is a three dimensional (3-D) map of electron density,absorptivity, complex index of refraction, or a combination of thesematerial characteristics. The map is generated using relatively fewphysical constraints. These techniques are described in further detailin U.S. Patent Publication No. 2015/0300965 by Sezginer et al., thesubject matter of which is incorporated herein by reference in itsentirety.

It is desirable to perform measurements at large ranges of angle ofincidence and azimuth angle to increase the precision and accuracy ofmeasured parameter values. This approach reduces correlations amongparameters by extending the number and diversity of data sets availablefor analysis to include a variety of large-angle, out of planeorientations. For example, in a normal orientation, T-SAXS is able toresolve the critical dimension of a feature, but is largely insensitiveto sidewall angle and height of a feature. However, by collectingmeasurement data over a broad range of out of plane angularorientations, the sidewall angle and height of a feature can beresolved. In other examples, measurements performed at large ranges ofangle of incidence and azimuth angle provide sufficient resolution anddepth of penetration to characterize high aspect ratio structuresthrough their entire depth.

Measurements of the intensity of diffracted radiation as a function ofx-ray incidence angle relative to the wafer surface normal arecollected. Information contained in the multiple diffraction orders istypically unique between each model parameter under consideration. Thus,x-ray scattering yields estimation results for values of parameters ofinterest with small errors and reduced parameter correlation.

Each orientation of the illuminating x-ray beam 116 relative to thesurface normal of a semiconductor wafer 101 is described by any twoangular rotations of wafer 101 with respect to the x-ray illuminationbeam 115, or vice-versa. In one example, the orientation can bedescribed with respect to a coordinate system fixed to the wafer. FIG. 2depicts x-ray illumination beam 116 incident on wafer 101 at aparticular orientation described by an angle of incidence, θ, and anazimuth angle, ϕ. Coordinate frame XYZ is fixed to the metrology system(e.g., illumination beam 116) and coordinate frame X′Y′Z′ is fixed towafer 101. The Y axis is aligned in plane with the surface of wafer 101.X and Z are not aligned with the surface of wafer 101. Z′ is alignedwith an axis normal to the surface of wafer 101, and X′ and Y′ are in aplane aligned with the surface of wafer 101. As depicted in FIG. 2,x-ray illumination beam 116 is aligned with the Z-axis and thus lieswithin the XZ plane. Angle of incidence, θ, describes the orientation ofthe x-ray illumination beam 116 with respect to the surface normal ofthe wafer in the XZ plane. Furthermore, azimuth angle, ϕ, describes theorientation of the XZ plane with respect to the X′Z′ plane. Together, θand ϕ, uniquely define the orientation of the x-ray illumination beam116 with respect to the surface of wafer 101. In this example, theorientation of the x-ray illumination beam with respect to the surfaceof wafer 101 is described by a rotation about an axis normal to thesurface of wafer 101 (i.e., Z′ axis) and a rotation about an axisaligned with the surface of wafer 101 (i.e., Y axis). In some otherexamples, the orientation of the x-ray illumination beam with respect tothe surface of wafer 101 is described by a rotation about a first axisaligned with the surface of wafer 101 and another axis aligned with thesurface of wafer 101 and perpendicular to the first axis.

In one aspect, wafer processing system 100 includes a specimenpositioning system 140 configured to actively position specimen 101 insix degrees of freedom with respect to illumination beam 116. Inaddition, specimen positioning system 101 is configured to alignspecimen 101 and orient specimen 101 over a large range of angles ofincidence (e.g., at least 70 degrees) and azimuth angle (e.g., at least190 degrees) with respect the illumination beam 116. In someembodiments, specimen positioning system 140 is configured to rotatespecimen 101 over a large range of angles of rotation (e.g., at least 70degrees) aligned in-plane with the surface of specimen 101. In thismanner, angle resolved measurements of specimen 101 are collected by thex-ray scatterometer over any number of locations and orientations on thesurface of specimen 101. In one example, computing system 130communicates command signals 139 to specimen positioning system 140 thatindicate the desired position of specimen 101. In response, specimenpositioning system 140 generates command signals to the variousactuators of specimen positioning system 140 to achieve the desiredpositioning of specimen 101.

In some other embodiments, the x-ray scatterometer system is configuredto rotate with respect to the wafer under measurement. In theseembodiments, the wafer is moved in the XY plane, and the opticalelements of the x-ray scatterometer are rotated about the point ofincidence of the illumination beam 116 on wafer 101.

The scattering efficiency of the measured specimen relates the extractedscattering intensities to the geometry and materials of the metrologytarget for a set of incidence angles {θ,ϕ}. FIG. 8 depicts a plot 153 ofthe scattering efficiency of the zero order beam, S₀₀, as a function ofangle of incidence, θ. S₀₀ depends on the incidence angle becausetransmission through the target decreases at higher incidence angles dueto increased path length. In addition, S₀₀ depends on the incidenceangle because energy leaves the zero order and enters the higherdiffracting orders when the incidence angle is aligned with thescattering of the target (e.g., normal incidence).

FIG. 9 depicts the scattering efficiency of several higher orders as afunction of angle of incidence, θ. Plotline 154 depicts S₁₁, plotline155 depicts S₁₃. plotline 156 depicts S₂₀, and plotline 157 depicts S₂₂.The scattering intensity for all higher orders typically depends on thescattering depth or density. In general, the scattering amplitude of thezero order decreases as scattering depth increases, while the scatteringamplitude of every other scattered order increases as scattering depthincreases.

In another aspect, process corrections are determined based on themeasured values of the parameters of interest (e.g., critical dimension,overlay, height, sidewall angle, etc.) and the corrections arecommunicated to the process tool to change one or more process controlparameters of the process tool (e.g., lithography tool, etch tool,deposition tool, etc.). In some embodiments, SAXS measurements areperformed and process control parameters are updated while the processis being executed on the measured structure. In some embodiments, SAXSmeasurements are performed after a particular process step and processcontrol parameters associated with that process step are updated forprocessing of future devices by that process step. In some embodiments,SAXS measurements are performed after a particular process step andprocess control parameters associated with a subsequent process step areupdated for processing of the measured device or other devices by thesubsequent process step.

In some examples, values of measured parameters determined based onmeasurement methods described herein can be communicated to an etch toolto adjust the etch time to achieve a desired etch depth. In a similarway etch parameters (e.g., etch time, diffusivity, etc.) or depositionparameters (e.g., time, concentration, etc.) may be included in ameasurement model to provide active feedback to etch tools or depositiontools, respectively. In some examples, corrections to process parametersdetermined based on measured device parameter values may be communicatedto the process tool. In one embodiment, computing system 130 determinesvalues of one or more parameters of interest during process based onmeasured signals 135 received from metrology system 101. In addition,computing system 130 communicates control commands 136 to processcontroller 109 based on the determined values of the one or moreparameters of interest. The control commands 136 cause the processcontroller 109 to change the state of the process (e.g., stop the etchprocess, change the diffusivity, etc.). In one example, control command136 causes process controller 109 to stop the etch process when adesired etch depth is measured. In another example, control command 136causes process controller 109 to change etch rate to improve measuredwafer uniformity of a CD parameter.

In general, as incident x-ray illumination interacts with periodicfeatures, the x-ray illumination scatters coherently to create adiffraction image on detector 119 (e.g., images 150-152 depicted inFIGS. 6-7). The desired scattered image or sequence of scattered imagesis achieved when the process tool is properly tuned. However, as themeasured images deviate from the desired image or sequence of desiredimages, these deviations indicate process tool drift and also thecorrections to process control variables required to bring the processtool back into proper tune.

FIG. 3 is a diagram illustrative of a semiconductor structure 141including two hole features undergoing an etch process. As depicted inFIG. 3, the initial profile 142 of the hole is changed to an enlargedprofile 143 by the etch process. As depicted in FIG. 3, x-rayillumination 116 is directed to structure 141 at the target of interest.This location of incidence is selected to best represent the aspects ofthe process critical to device yield. As the etch process progresses,the depth of the hole features and the width of the hole featureschange.

FIG. 4 is a diagram illustrative of a semiconductor structure 144including two hole features undergoing a deposition process. As depictedin FIG. 4, the initial profile 145 of the hole is changed to a reducedprofile 145 by the deposition process. As depicted in FIG. 4, x-rayillumination 116 is directed to structure 144 at the target of interest.This location of incidence is selected to best represent the aspects ofthe process critical to device yield. As the deposition processprogresses, the depth of the hole features and the width of the holefeatures change.

FIG. 6 depicts scattering images 150 and 151 illustrative of changes ofthe size of an etched hole. As the etched hole feature changes sizeduring the etch process, the spatial Fourier transform of the etchedfeature changes causing a change in the diffraction pattern. As theintensity pattern across the orders shrinks, it indicates that thefeature size is increasing (e.g., the diameter of a hole feature isincreasing). To avoid an unwanted increase in hole diameter, a processcontrol parameter (e.g., etch time) is adjusted to prevent unwantedincrease in hole diameter.

FIG. 7 depicts scattering images 150 and 152 illustrative of changes ofthe depth of an etched hole. As the etched hole deepens during an etchprocess or becomes more shallow during a deposition process, the spatialFourier transform of the etched feature changes causing a change in theintensity of the detected diffraction pattern. As the intensity acrossthe orders increases, it indicates that the hole depth is increasing, orvice versa, as the intensity across the orders decreases, it indicatesthat the hole depth is decreasing. To avoid an unwanted change in holedepth, a process control parameter (e.g., etch time, bias voltage, etc.)is adjusted to prevent unwanted change in hole depth.

FIG. 5 is a diagram illustrative of a semiconductor structure 147including two layers, each including two hole features. The first layerincludes a filled hole 148. The second layer includes an unfilled hole149. As depicted in FIG. 5, hole 149 of the second layer is tilted andoffset by alignment error distance, A_(E), with respect to filled hole148 of the first layer. As depicted in FIG. 5, x-ray illumination 116 isdirected to structure 147 at the target of interest. This location ofincidence is selected to best represent the aspects of the processcritical to device yield.

The phase of the detected scattering from the lower structure (i.e.,hole 148) with respect to the processed structure (i.e., hole 149)provides an indication of the alignment error distance, A_(E).Scattering from the processed structure with respect to the incidenceangle of the x-ray illumination 116 provides an indication of the tiltof hole 149. Together, these measured signals enable the estimation ofoverlay error and tilt. In turn, edge placement errors are corrected bycorrecting process control parameters of an etch tool to correct tiltand correcting process control parameters of a lithography tool tocorrect overlay error. Additional description of monitoring of overlayand hole tilt is provided in U.S. Patent Application No. 2015/0117610,the subject matter of which is incorporated herein by reference it itsentirety.

In general, a metrology target is characterized by an aspect ratiodefined as a maximum height dimension (i.e., dimension normal to thewafer surface) divided by a maximum lateral extent dimension (i.e.,dimension aligned with the wafer surface) of the metrology target. Insome embodiments, the metrology target under measurement has an aspectratio of at least twenty. In some embodiments, the metrology target hasan aspect ratio of at least forty.

FIGS. 12A-12C depict an isometric view, a top view, and across-sectional view, respectively, of a typical 3D FLASH memory device170 subject to measurement in the manner described herein. The totalheight (or equivalently depth) of memory device 170 ranges from one toseveral micrometers. Memory device 170 is a vertically manufactureddevice. A vertically manufactured device, such as memory device 170,essentially turns a conventional, planar memory device 90 degrees,orienting the bit line and cell string vertically (perpendicular towafer surface). To provide sufficient memory capacity, a large number ofalternating layers of different materials are deposited on the wafer.This requires patterning processes to perform well to depths of severalmicrons for structures with a maximum lateral extent of one hundrednanometers or less. As a result, aspect ratios of 25 to 1 or 50 to 1 arenot uncommon.

FIG. 10 depicts a top view of an array of high aspect ratio holestructures 160. As depicted in FIG. 10, the array of hole structures ismost closely patterned along planes 161, 162, 163, and 164 (which extendinward and outward from the drawing). In some embodiments, it ispreferred to perform measurements of high aspect ratio structures asdescribed herein at orientations of the incident x-ray illumination beamwith respect to the surface of the wafer under measurement that liewithin planes where an array of high aspect ratio structures is mostclosely patterned. In the example depicted in FIG. 10, it is preferredto provide x-ray illumination to the array of hole structures 160 withinplanes 161, 162, 163, and 164, where the array of hole structures ismost closely patterned.

FIG. 11A depicts a side view of an ideal high aspect ratio holestructure 165. FIG. 11B depicts a side view of a tilted hole structure166. FIG. 11C depicts a side view of a progressively tilted holestructure 167, where the degree of tilt progressively increases withdepth. In many examples, hole structures 166 and 167 are undesirable. Insome embodiments, hole structures resembling hole structures 166 and 167are characterized by T-SAXS measurements as described herein. In oneexample, hole structure 166 is characterized by a tilt angle parameter,α. Furthermore, x-ray illumination beam 116 is provided to holestructure 166 at an angle, ϕ, with respect to the surface normal, and atthe opposite angle, −ϕ, as described, for example, with reference toFIG. 2. In some embodiments, differences in measured T-SAX signals thatarise in these two illumination scenarios provide sufficient signalinformation to accurately estimate the tilt angle, α.

In another example, hole structure 167 is piecewise characterized by anumber of tilt angle parameter, α₁, α₂, and α₃. Similarly, x-rayillumination beam 116 is provided to hole structure 167 at an angle, ϕ,with respect to the surface normal, and at the opposite angle, −ϕ, asdescribed, for example, with reference to FIG. 2. In some embodiments,differences in measured T-SAX signals that arise in these twoillumination scenarios provide sufficient signal information toaccurately estimate the tilt angles, α₁, α₂, and α₃.

In the embodiment depicted in FIG. 1, the T-SAXS metrology system isintegrated with the process tool, and provides measurement results tothe process tool while the process is undertaken. However, in general,the T-SAXS metrology system may be implemented as a standalone tool. Inthese embodiments, the process step must be completed and wafer 101transferred to the standalone T-SAXS tool for measurement. Changes toprocess control variables are communicated to the process tool forprocessing of subsequent wafers.

The measurement frequency of a particular high aspect ratio structureunder fabrication depends on the stability of the monitored processparameter. Moreover, the length of measurement time required depends onthe scattering sensitivity to changes in the monitored process variable.Measuring a scattering signal in-situ provides the fastest measurementof process conditions but with the highest degree of uncertainty.Whereas, longer measurement times provide greater precision andcertainty of the measured parameters. In general, process parametersthat affect the entire scattering volume (e.g., etch time) can bemonitored the fastest, while other parameters (e.g., minor deviations ofthe etch profile) require either a moving average or longer measurementtime to achieve meaningful results. Thus, these parameters can only becontrolled on a slower basis. The tradeoff between sensitivity andrequired measurement time exists whether the measurements are performedin-situ or on a standalone tool. However, longer measurements aretypically more accurately performed in a more stable, standalone tooldesigned for that particular measurement.

Although, FIG. 1 depicts a transmission SAXS measurement system, ingeneral, a reflective SAXS measurement system may be employed to measureshallow features.

FIG. 13 depicts an exemplary wafer processing system 200 for monitoringof an etch process based on x-ray scatterometry measurements ofsemiconductor structures disposed on a wafer under process. In oneaspect, a reflective scatterometry metrology system is integrated withan etch process tool. The measured values of the parameters of interestare provided as feedback to control the etch process tool.

Wafer processing system 200 includes a process chamber 204 containing aprocess environment 203 and a reflective x-ray scatterometer.Semiconductor wafer 201 is located within process chamber 204. Wafer 201is attached to wafer chuck 205 and is positioned with respect to processchamber 204 and the x-ray scatterometer by wafer stage 240.

In one embodiment, process chamber 204 is an element of a reactive ionetch system. In this embodiment, process environment 203 includes aradio frequency induced plasma that etches away exposed material on thesurface of wafer 201.

In the depicted embodiment, the SAXS metrology system includes an x-rayillumination source 210 configured to generate x-ray radiation suitablefor reflective SAXS measurements analogous to the description ofillumination source 110 with reference to FIG. 1.

In some examples, computing system 130 communicates command signals 237to x-ray illumination source 210 that cause x-ray illumination source210 to emit x-ray radiation at a desired energy level. The energy levelis changed to acquire measurement data with more information about thehigh aspect ratio structures under measurement.

The illumination beam 216 passes through window 206 of process chamber204 and illuminations specimen 201 over a measurement spot 201. Afterincidence with wafer 201, scattered x-ray radiation 214 exits processchamber 204 through window 207. In some embodiments, the optical pathlength between process chamber 204 and detector 219 (i.e., thecollection beam path) is long and x-ray scattering in air contributesnoise to the image on the detector. Hence, in preferred embodiments, asignificant portion of the collection beam path length between processchamber 204 and detector 219 is maintained in a localized vacuumenvironment.

X-ray detector 219 collects x-ray radiation 214 scattered from specimen201 and generates an output signals 235 indicative of properties ofspecimen 201 that are sensitive to the incident x-ray radiation inaccordance with a reflective SAXS measurement modality. In someembodiments, scattered x-rays 214 are collected by x-ray detector 219while specimen positioning system 240 locates and orients specimen 201to produce angularly resolved scattered x-rays in accordance withcommand signals 239 communicated from computing system 230 to specimenpositioning system 240.

In a further aspect, computing system 230 is employed to determineproperties of wafer 201 (e.g., structural parameter values) based on oneor more diffraction orders of scattered light. As depicted in FIG. 13,system 200 includes a computing system 230 employed to acquire signals235 generated by detector 219 and determine properties of the specimenbased at least in part on the acquired signals, and store an indication222 of the determined values of the parameters of interest in a memory(e.g., memory 290). In some embodiments, computing system 230 isconfigured as a process control metrology engine to directly estimatevalues of one or more parameters of interest based on scatterometrymeasurements of wafers under process using a measurement model.

In addition, computing system 130 communicates control commands 236 toprocess controller 209 based on the determined values of the one or moreparameters of interest. The control commands 236 cause the processcontroller 209 to change the state of the process (e.g., stop the etchprocess, change the diffusivity, etc.).

FIG. 14 is a diagram illustrative of an exemplary model building andanalysis engine 180 implemented by computing system 130. As depicted inFIG. 14, model building and analysis engine 180 includes a structuralmodel building module 181 that generates a structural model 182 of ameasured structure of a specimen. In some embodiments, structural model182 also includes material properties of the specimen. The structuralmodel 182 is received as input to T-SAXS response function buildingmodule 183. T-SAXS response function building module 183 generates aT-SAXS response function model 184 based at least in part on thestructural model 182. In some examples, the T-SAXS response functionmodel 183 is based on x-ray form factors,

$\begin{matrix}{{F\left( \overset{\rho}{q} \right)} = {\int{{\rho\left( \overset{\rho}{r} \right)}e^{{- i}{\overset{\rho}{q} \cdot \overset{\rho}{r}}}d{\overset{\rho}{r}}^{\;}}}} & (1)\end{matrix}$

where F is the form factor, q is the scattering vector, and ρ(r) is theelectron density of the specimen in spherical coordinates. The x-rayscattering intensity is then given byI({right arrow over (q)})=F*F.  (2)

T-SAXS response function model 184 is received as input to fittinganalysis module 185. The fitting analysis module 185 compares themodeled T-SAXS response with the corresponding measured data 135 todetermine geometric as well as material properties of the specimen.

In some examples, the fitting of modeled data to experimental data isachieved by minimizing a chi-squared value. For example, for T-SAXSmeasurements, a chi-squared value can be defined as

$\begin{matrix}{\chi_{SAXS}^{2} = {\frac{1}{N_{SAXS}}{\sum\limits_{j}^{N_{SAXS}}\frac{\left( {{S_{j}^{{SAXS}\mspace{11mu}{model}}\left( {v_{1},\ldots\mspace{14mu},v_{L}} \right)} - S_{j}^{{SAXS}\mspace{11mu}{experiment}}} \right)^{2}}{\sigma_{{SAXS},j}^{2}}}}} & (3)\end{matrix}$

Where, S_(j) ^(SAXS experiment) is the measured T-SAXS signals 126 inthe “channel” j, where the index j describes a set of system parameterssuch as diffraction order, energy, angular coordinate, etc. S_(j)^(SAXS model)(v₁, . . . v_(L)) is the modeled T-SAXS signal S_(j) forthe “channel” j, evaluated for a set of structure (target) parametersv₁, . . . , v_(L), where these parameters describe geometric (CD,sidewall angle, overlay, etc.) and material (electron density, etc.).σ_(SAXS,j) is the uncertainty associated with the jth channel. N_(SAXS)is the total number of channels in the x-ray metrology. L is the numberof parameters characterizing the metrology target.

Equation (3) assumes that the uncertainties associated with differentchannels are uncorrelated. In examples where the uncertaintiesassociated with the different channels are correlated, a covariancebetween the uncertainties, can be calculated. In these examples achi-squared value for T-SAXS measurements can be expressed as

$\begin{matrix}{\chi_{SAXS}^{2} = {\frac{1}{N_{SAXS}}\left( {{{\overset{\rightarrow}{S}}_{j}^{{SAXS} \cdot {model}}\left( {v_{1},\ldots\mspace{14mu},v_{M}} \right)} - {\overset{\rightarrow}{S}}_{j}^{{SAXS} \cdot {experiment}}} \right)^{T}{V_{SAXS}^{- 1}\left( {{{\overset{\rightarrow}{S}}_{j}^{{SAXS} \cdot {model}}\left( {v_{1},\ldots\mspace{14mu},v_{M}} \right)} - {\overset{\rightarrow}{S}}_{j}^{{SAXS} \cdot {experiment}}} \right)}}} & (4)\end{matrix}$

where, V_(SAXS) is the covariance matrix of the SAXS channeluncertainties, and T denotes the transpose.

In some examples, fitting analysis module 185 resolves at least onespecimen parameter value by performing a fitting analysis on T-SAXSmeasurement data 135 with the T-SAXS response model 184. In someexamples, X_(SAXS) ² is optimized.

As described hereinbefore, the fitting of T-SAXS data is achieved byminimization of chi-squared values. However, in general, the fitting ofT-SAXS data may be achieved by other functions.

The fitting of T-SAXS metrology data is advantageous for any type ofT-SAXS technology that provides sensitivity to geometric and/or materialparameters of interest. Specimen parameters can be deterministic (e.g.,CD, SWA, etc.) or statistical (e.g., rms height of sidewall roughness,roughness correlation length, etc.) as long as proper models describingT-SAXS beam interaction with the specimen are used.

In general, computing system 130 is configured to access modelparameters in real-time, employing Real Time Critical Dimensioning(RTCD), or it may access libraries of pre-computed models fordetermining a value of at least one specimen parameter value associatedwith the specimen 101. In general, some form of CD-engine may be used toevaluate the difference between assigned CD parameters of a specimen andCD parameters associated with the measured specimen. Exemplary methodsand systems for computing specimen parameter values are described inU.S. Pat. No. 7,826,071, issued on Nov. 2, 2010, to KLA-Tencor Corp.,the entirety of which is incorporated herein by reference.

In some examples, model building and analysis engine 180 improves theaccuracy of measured parameters by any combination of feed sidewaysanalysis, feed forward analysis, and parallel analysis. Feed sidewaysanalysis refers to taking multiple data sets on different areas of thesame specimen and passing common parameters determined from the firstdataset onto the second dataset for analysis. Feed forward analysisrefers to taking data sets on different specimens and passing commonparameters forward to subsequent analyses using a stepwise copy exactparameter feed forward approach. Parallel analysis refers to theparallel or concurrent application of a non-linear fitting methodologyto multiple datasets where at least one common parameter is coupledduring the fitting.

Multiple tool and structure analysis refers to a feed forward, feedsideways, or parallel analysis based on regression, a look-up table(i.e., “library” matching), or another fitting procedure of multipledatasets. Exemplary methods and systems for multiple tool and structureanalysis is described in U.S. Pat. No. 7,478,019, issued on Jan. 13,2009, to KLA-Tencor Corp., the entirety of which is incorporated hereinby reference.

In another aspect, one or more SAXS systems integrated with a processtool are configured to measure multiple, different areas of a waferduring a process interval. In some embodiments, a wafer uniformity valueassociated with each measured parameter of interest is determined basedon measured values of each parameter of interest across the wafer.

In some embodiments, multiple metrology systems are integrated with theprocess tool and the metrology systems are configured to simultaneouslymeasure different areas across the wafer during process. In someembodiments, a single metrology system integrated with a process tool isconfigured to sequentially measure multiple, different areas of a waferduring process.

In some embodiments, the methods and systems for SAXS based metrology ofsemiconductor devices undergoing a process as described herein areapplied to the measurement of memory structures. These embodimentsenable critical dimension (CD), film, and composition metrology forperiodic and planar structures.

Scatterometry measurements as described herein may be used to determinecharacteristics of a variety of semiconductor structures. Exemplarystructures include, but are not limited to, FinFETs, low-dimensionalstructures such as nanowires or graphene, sub 10 nm structures,lithographic structures, through substrate vias (TSVs), memorystructures such as DRAM, DRAM 4F2, FLASH, MRAM and high aspect ratiomemory structures. Exemplary structural characteristics include, but arenot limited to, geometric parameters such as line edge roughness, linewidth roughness, pore size, pore density, side wall angle, profile,critical dimension, pitch, thickness, overlay, and material parameterssuch as electron density, composition, grain structure, morphology,stress, strain, and elemental identification. In some embodiments, themetrology target is a periodic structure. In some other embodiments, themetrology target is aperiodic.

In some examples, measurements of critical dimensions, thicknesses,overlay, and material properties of high aspect ratio semiconductorstructures including, but not limited to, spin transfer torque randomaccess memory (STT-RAM), three dimensional NAND memory (3D-NAND) orvertical NAND memory (V-NAND), dynamic random access memory (DRAM),three dimensional FLASH memory (3D-FLASH), resistive random accessmemory (Re-RAM), and phase change random access memory (PC-RAM) areperformed with T-SAXS measurement systems as described herein.

In some examples, the measurement models are implemented as an elementof a SpectraShape® critical-dimension metrology system available fromKLA-Tencor Corporation, Milpitas, Calif., USA. In this manner, the modelis created and ready for use immediately after the scattering images arecollected by the system.

In some other examples, the measurement models are implemented off-line,for example, by a computing system implementing AcuShape® softwareavailable from KLA-Tencor Corporation, Milpitas, Calif., USA. Theresulting models may be incorporated as an element of an AcuShape®library that is accessible by a metrology system performingmeasurements.

FIG. 15 illustrates a method 300 of performing metrology measurementsduring process in at least one novel aspect. Method 300 is suitable forimplementation by a metrology system such as the SAXS metrology systemsillustrated in FIGS. 1 and 13 of the present invention. In one aspect,it is recognized that data processing blocks of method 300 may becarried out via a pre-programmed algorithm executed by one or moreprocessors of computing system 130, computing system 230, or any othergeneral purpose computing system. It is recognized herein that theparticular structural aspects of the metrology systems depicted in FIGS.1 and 13 do not represent limitations and should be interpreted asillustrative only.

In block 301, an amount of x-ray illumination light is provided to ameasurement spot including one or more high aspect ratio structurespartially fabricated on a semiconductor wafer.

In block 302, an amount of x-ray light reflected from or transmittedthrough the semiconductor wafer is detected in response to the amount ofx-ray illumination light.

In block 303, values of one or more parameters of interest associatedwith the partically fabricated one or more high aspect ratio structuresare determined based on the detected amount of x-ray light.

In block 304, an indication of the values of the one or more parametersof interest is communicated to a fabrication tool that causes thefabrication tool to adjust a value of one or more process controlparameters of the fabrication tool.

In a further embodiment, system 100 includes one or more computingsystems 130 employed to perform measurements of semiconductor structuresbased on scatterometry measurement data collected in accordance with themethods described herein. The one or more computing systems 130 may becommunicatively coupled to one or more detectors, active opticalelements, process controllers, etc. In one aspect, the one or morecomputing systems 130 are configured to receive measurement dataassociated with scatterometry measurements of structures of wafer 101.

It should be recognized that one or more steps described throughout thepresent disclosure may be carried out by a single computer system 130or, alternatively, a multiple computer system 130. Moreover, differentsubsystems of system 100 may include a computer system suitable forcarrying out at least a portion of the steps described herein.Therefore, the aforementioned description should not be interpreted as alimitation on the present invention but merely an illustration.

In addition, the computer system 130 may be communicatively coupled tothe spectrometers in any manner known in the art. For example, the oneor more computing systems 130 may be coupled to computing systemsassociated with the scatterometers. In another example, thescatterometers may be controlled directly by a single computer systemcoupled to computer system 130.

The computer system 130 of system 100 may be configured to receiveand/or acquire data or information from the subsystems of the system(e.g., scatterometers and the like) by a transmission medium that mayinclude wireline and/or wireless portions. In this manner, thetransmission medium may serve as a data link between the computer system130 and other subsystems of system 100.

Computer system 130 of system 100 may be configured to receive and/oracquire data or information (e.g., measurement results, modeling inputs,modeling results, etc.) from other systems by a transmission medium thatmay include wireline and/or wireless portions. In this manner, thetransmission medium may serve as a data link between the computer system130 and other systems (e.g., memory on-board system 100, externalmemory, or other external systems). For example, the computing system130 may be configured to receive measurement data from a storage medium(i.e., memory 132 or an external memory) via a data link. For instance,scattered images obtained using the scatterometers described herein maybe stored in a permanent or semi-permanent memory device (e.g., memory132 or an external memory). In this regard, the scatterometry images maybe imported from on-board memory or from an external memory system.Moreover, the computer system 130 may send data to other systems via atransmission medium. For instance, a measurement model or an estimatedparameter value determined by computer system 130 may be communicatedand stored in an external memory. In this regard, measurement resultsmay be exported to another system.

Computing system 130 may include, but is not limited to, a personalcomputer system, mainframe computer system, workstation, image computer,parallel processor, or any other device known in the art. In general,the term “computing system” may be broadly defined to encompass anydevice having one or more processors, which execute instructions from amemory medium.

Program instructions 134 implementing methods such as those describedherein may be transmitted over a transmission medium such as a wire,cable, or wireless transmission link. For example, as illustrated inFIG. 1, program instructions 134 stored in memory 132 are transmitted toprocessor 131 over bus 133. Program instructions 134 are stored in acomputer readable medium (e.g., memory 132). Exemplary computer-readablemedia include read-only memory, a random access memory, a magnetic oroptical disk, or a magnetic tape. Computing system 230, includingelements 231-234, is analogous to computing system 130, includingelements 131-134, respectively, as described herein.

As described herein, the term “critical dimension” includes any criticaldimension of a structure (e.g., bottom critical dimension, middlecritical dimension, top critical dimension, sidewall angle, gratingheight, etc.), a critical dimension between any two or more structures(e.g., distance between two structures), and a displacement between twoor more structures (e.g., overlay displacement between overlayinggrating structures, etc.). Structures may include three dimensionalstructures, patterned structures, overlay structures, etc.

As described herein, the term “critical dimension application” or“critical dimension measurement application” includes any criticaldimension measurement.

As described herein, the term “metrology system” includes any systememployed at least in part to characterize a specimen in any aspect,including measurement applications such as critical dimension metrology,overlay metrology, focus/dosage metrology, and composition metrology.However, such terms of art do not limit the scope of the term “metrologysystem” as described herein. In addition, the metrology system may beconfigured for measurement of patterned wafers and/or unpatternedwafers. The metrology system may be configured as a LED inspection tool,edge inspection tool, backside inspection tool, macro-inspection tool,or multi-mode inspection tool (involving data from one or more platformssimultaneously), and any other metrology or inspection tool thatbenefits from the calibration of system parameters based on criticaldimension data.

Various embodiments are described herein for a semiconductor measurementsystem that may be used for measuring a specimen within anysemiconductor processing tool (e.g., an inspection system or alithography system). The term “specimen” is used herein to refer to awafer, a reticle, or any other sample that may be processed (e.g.,printed or inspected for defects) by means known in the art.

As used herein, the term “wafer” generally refers to substrates formedof a semiconductor or non-semiconductor material. Examples include, butare not limited to, monocrystalline silicon, gallium arsenide, andindium phosphide. Such substrates may be commonly found and/or processedin semiconductor fabrication facilities. In some cases, a wafer mayinclude only the substrate (i.e., bare wafer). Alternatively, a wafermay include one or more layers of different materials formed upon asubstrate. One or more layers formed on a wafer may be “patterned” or“unpatterned.” For example, a wafer may include a plurality of dieshaving repeatable pattern features.

A “reticle” may be a reticle at any stage of a reticle fabricationprocess, or a completed reticle that may or may not be released for usein a semiconductor fabrication facility. A reticle, or a “mask,” isgenerally defined as a substantially transparent substrate havingsubstantially opaque regions formed thereon and configured in a pattern.The substrate may include, for example, a glass material such asamorphous SiO₂. A reticle may be disposed above a resist-covered waferduring an exposure step of a lithography process such that the patternon the reticle may be transferred to the resist.

One or more layers formed on a wafer may be patterned or unpatterned.For example, a wafer may include a plurality of dies, each havingrepeatable pattern features. Formation and processing of such layers ofmaterial may ultimately result in completed devices. Many differenttypes of devices may be formed on a wafer, and the term wafer as usedherein is intended to encompass a wafer on which any type of deviceknown in the art is being fabricated.

In one or more exemplary embodiments, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by ageneral purpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

What is claimed is:
 1. An x-ray scatterometry based metrology systemcomprising: an x-ray illumination source configured to provide an amountof x-ray illumination light directed to a measurement spot including oneor more structures partially fabricated on a semiconductor wafer; adetector configured to detect an amount of x-ray light reflected from ortransmitted through the semiconductor wafer in response to the amount ofx-ray illumination light; and a computing system configured to:determine values of one or more parameters of interest associated withthe partially fabricated one or more structures based on the detectedamount of x-ray light; and communicate an indication of the values ofthe one or more parameters of interest to a fabrication tool that causesthe fabrication tool to adjust a value of one or more process controlparameters of the fabrication tool, wherein the one or more structurespartially fabricated on the semiconductor wafer are fabricated at leastin part by the fabrication tool.
 2. The x-ray scatterometry basedmetrology system of claim 1, wherein the x-ray illumination sourceprovides the amount of x-ray illumination light and the detector detectsthe amount of x-ray light while the fabrication tool is fabricating theone or more structures.
 3. The x-ray scatterometry based metrologysystem of claim 2, wherein the adjusting of the value of the one or moreprocess control parameters occurs while the fabrication tool isfabricating the one or more high aspect ratio structures.
 4. The x-rayscatterometry based metrology system of claim 2, wherein the x-rayillumination source and the detector are integrated with the fabricationtool as part of a semiconductor fabrication system.
 5. The x-rayscatterometry based metrology system of claim 4, the fabrication toolcomprising a fabrication process chamber comprising a fabricationprocess environment, the semiconductor wafer disposed inside thefabrication process chamber and exposed to the fabrication processenvironment during a process interval, wherein the x-ray illuminationsource provides the amount of x-ray illumination light and the detectordetects the amount of x-ray light during the process interval.
 6. Thex-ray scatterometry based metrology system of claim 1, wherein the x-rayillumination source provides the amount of x-ray illumination light andthe detector detects the amount of x-ray light after the fabricationtool has completed a fabrication step.
 7. The x-ray scatterometry basedmetrology system of claim 1, wherein the values of the one or moreparameters of interest are determined at a first process step, andwherein the indication of the values of the one or more parameters ofinterest communicated to the fabrication tool cause the fabrication toolto adjust a value of one or more process control parameters of thefabrication tool at a second process step subsequent to the firstprocess step in a fabrication process flow of the one or morestructures.
 8. The x-ray scatterometry based metrology system of claim1, wherein the values of the one or more parameters of interest aredetermined at a process step in a fabrication process flow of the one ormore structures, and wherein the indication of the values of the one ormore parameters of interest communicated to the fabrication tool causethe fabrication tool to adjust a value of one or more process controlparameters of the fabrication tool at the process step.
 9. The x-rayscatterometry based metrology system of claim 1, wherein the value ofone or more process control parameters of the fabrication tool controlsany of an etch process, a deposition process, and a lithography process.10. The x-ray scatterometry based metrology system of claim 1, whereinthe amount of x-ray illumination light is directed to the measurementspot at a plurality of angles of incidence, azimuth angles, or both. 11.The x-ray scatterometry based metrology system of claim 1, wherein thex-ray illumination source is further configured to provide the amount ofx-ray illumination light directed to a measurement spot at a pluralityof different energy levels.
 12. The x-ray scatteromery based metrologysystem of claim 1, wherein the determining the values of the one or moreparameters of interest is based on a model-based measurement model, atrained signal response metrology (SRM) measurement model, or atomographic measurement model.
 13. The x-ray scatterometry basedmetrology system of claim 1, wherein the one or more structures includesa three-dimensional NAND structure or a dynamic random access memory(DRAM) structure.